I determined which balsa rib on the wing, will act as one of the side walls to the bay and from there, I simply placed the rib over a good sized plastic and cut around the rib. Next, I laminated a piece of plastic to the side that will face the inside of the bay. This way, I was able to set the boundaries of the machine gun bay and the walls.
My fingers were too big and thick to place more walls (see the first picture of my fingers holding onto the rib), so I used a pair of tweezers to add more inner walls with thin plastic as the next photo shows.,
As the last picture shows, this wall is too high. It will stick through the covering. As you can see, there is a line drawn with a pen showing me where to make the cut on the plastic to keep the wall within height. As you can see I had to make the cut a taper from the root (which is to the left of this picture) out to the tip which is to the right of this picture. I had to lower my head to the same height as the wing and "eyeball" the correct height.